Use of tape-controlled milling machines to carve combustible casting patterns

ABSTRACT

A tape-controlled milling machine, adjusted for material removal at a rate at least 100 times as great as for steel, is used to carve casting patterns from expanded polystyrene. Billets of the polystyrene material are carved without repetitive passes. Each rotatable cutter utilized is directed by the program of the tape to bring its edge through the excess material of the billet all the way to the final surface to be carved, and then to proceed along the final contour to be cut by it. Cost savings, material even where a single combustible pattern is to be produced, are multiplied when the tape is reused.

Mutt tts em [15] mama Parsons I [45] ,l u all l la' W72 [54] USE or'llE-CQNTROLLED LING 3,370,508 2/1968 laia ..90/11 0 MACHINES T0 @ARVElCOMlBUS l l @ASTmG PAWERNS Primary ExaminerGil WeidenfeldAttorney-Jerome A. Gross [72] Inventor: John T. Parsons, 205 Wellington,Traverse City, Mich. 49634 57 33 CT [22] Filed 1969 A tape-controlledmilling machine, adjusted for material [2 1] Appl' NM 867,989 removal ata rate at least 100 times as great as for steel, is used to carvecasting patterns from expanded polystyrene. Billets of the polystyrenematerial are carved without repetitive passes.

"90/11 90/13 agga 3 Each rotatable cutter utilized is directed by theprogram of the 53] w 1 l 3 l 3 8 tape to bring its edge through theexcess material of the billet all the way to the final surface to becarved, and then to [55 References Cited proceed along the final contourto be cut by it. Cost savings,

material even where a single combustible pattern is to be UNITED STATESPATENTS produced, are multiplied when the tape is reused.

3,002,115 9/1961 Johnson et al ..90/l3 C X 5 Claims, 2 Drawing FigureslUSlE ill TAPE-CONTROLLED MILLING MACHINES TO CARI/E COIVIRUSTIBILIECASTING IATTEIRNfi BACKGROUND OF THE INVENTION The present inventionrelates both to material removal by tape-controlled milling machines,and to the formation of combustible casting patterns, as from expandedpolystyrene.

Tape-controlled milling machines have heretofore been designed formilling steel and other metals according to taped programs which controlthe operations of the machine, determining the depth of cut and thedirection and rate of movement of the cutter relative to the material.They are particularly valuable where a number of identical parts are tobe milled; but even for making a single part, they are to be preferredwhen a skilled programmer is available. In machining ordinary steel, therate of material removal is a function in part of the horsepoweravailable at the spindle or spindles of the machine. When machiningother materials, for example aluminum and other soft metals, the rate ofmaterial removal is not increased inversely according to the density ofthe material to be removed; although softer than steel, such othermetals may have other qualities which interfere with theirmachineability. Likewise, in dealing with materials such as wood, onewould not expect to remove material at a rate, compared with the removalrate of steel, inversely proportional to the comparative densities ofwood to steel; because the aligned fibrous structure of the wood wouldnot permit such a rapid removal without shattering.

In the art of making combustible patterns using expanded polystyrene,patternmakers have been employed; and they have followed conventionalpractices for making wood patterns. Thus, boards are cut from billets ofthe expanded polystyrene material and are glued together. When filletsare made, they are usually made as a wooden fillet would be made, thatis, by cutting a strip of square cross section on the diagonal, andgluing it in place. The amount of work necessary to form an arcuatefillet is considered prohibitive. However, wax has been used, both as afillet material and as a filler between imperfectly fitting polystyrenemembers glued together to form a pattern. Such construction of expandedpolystyrene patterns is shown in U.S. Pat. No. 2,830,343. The amounts ofglue and wax must be minimized, so that combustion of the pattern is notimpaired; hence if patterns are of complex form, they are likely to bepoorly assembled and badly aligned. Where a number of identical castingsare to be produced, the cost of patternmaking is multiplied.

SUMMARY OF THE INVENTION The purposes of the present invention includeeliminating the laborious and expensive process by which combustiblepatterns, such as those formed of expanded polystyrene, are madepainstakingly in the manner of wooden patterns; and to provide patternsof much greater accuracy, including integrally formed arcuate filletsmade without pattemmaking skills and at a high rate of speed, and in aslarge quantities as may be desired.

The present invention may be briefly summarized as a new use fortape-controlled milling machines, in which a method is provided ofcarving the type of low-density casting patterns which are combustibleunder a charge of molten cast metal. The preferred material is expandedpolystyrene whose density is less than 3 pounds per cubic foot. In thepresent method, the machine is provided with a milling cutter,preferably of the type having the capacity to cut to a depth more thanfive times the feasible depth of cut when machining steel. The millingmachine is adjusted, or rebuilt if necessary, to multiply depth of cut,linear feed, rate and speed of cutter rotation so as to achieve a rateof material removal at least 100 times as great as for steel. After abillet of the combustible pattern material is chucked in the machine,the tape directing the program of machine operations causes the desiredpattern to be quickly carved. Regardless of the size of the billet, eachcutter is directed through the excess billet material immediately to aposition at which its cutting edge may carve a final surface portion ofthe pattern. Where the cutter edge to be utilized is a side edge, thecutter is so directed that this edge is brought into the billet to aposition along the desired final contour of the pattern; thereafter thecutter edge follows a final contour portion, carving away all excessbillet material. Where a depressed surface is to be carved transverse tothe cutter spindles using a transverse edge as the cutter tip, thecutter is plunged axially into the billet until its tip reaches thedepth at which such surface is to be carved, and its tip-cutting edgeportion is then directed along a final contour portion of such surface.Meanwhile the tool portion thereabove removes the billet materialthereabove over the full width of the cutter and the full depth to whichit has been so plunged.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a fragmentary view of abillet of expanded polystyrene material, illustrated in the course ofbeing cut partly to final contour by a first cutter of a tape-controlledmilling machine (not shown).

FIG. 2 is a view similar to FIG. 11 showing the subsequent carving, by asecond milling cutter, of remaining surface portions to final contour.

DESCRIPTION OF THE PREFERRED EMBODIMENT The final product of the presentinvention is an integral casting pattern, generally designated 10,formed of low-density combustible pattern material to the relativelycomplex con tours shown fragmentarily in FIG. 2. Expanded polystyrene ofa density of between 1 and 3 pounds per cubic foot is preferred. Thespecific pattern illustrated is of hollow boxlike shape, having a squarecut end wall 11 and parallel sidewalls 12 including thinned sections 13joining the walls 112 at very large radius internal fillets 141 formedabout axes b. The pattern 10 also has a bottom wall 15 including cutoutsl6 flanked by small radius fillets 17 to be described, and leadingupward in a step to a thickened bottom wall Id. A second small radiusfillet 19 joins the bottom wall 118 to the inner vertical wall surfaces20 of cavities out within the pattern 10. In the embodiment shown, aplurality of such cavities is formed by cross webs 21 whose thicknessapproximately equals that of the thinned wall sections 113. The walls,Ill, 12 and webs 21 of the pattern 10 have a horizontal planar upperedge surface 21.

The vertical inner surfaces 20 of the several cavities of the pattern 10flow smoothly into each other at internal arcuate fillets 25 of generousradius somewhat smaller than the external fillets l4 and formed aboutvertical axes d. The smaller radius fillets l7, l9 merge with the largerradius fillets 25 at the lower inside comers of such cavities.

In order to form the casting pattern according to the method of thepresent invention, l commence with a rectangular billet of the expandedpolystyrene material, cut to the length, width and depth shown. I firstform the roughly carved article generally designated 30 shown in FIG.11. To do so I chuck the billet in a tape-controlled milling machine,whose vertical driving spindle 311 is shown fragmentarily in thedrawings. The milling machine utilized is of a conventionaltype designedto machine steel, and the horsepower delivered to the driving spindle 31is such as will remove steel alloys, for example, those conventionallyused in casting, at a known or determinable rate of material removal.

In carrying out the present invention, I so adjust or modify the speedof the milling machine, changing in its controls and lead screws ifnecessary, to effect a rate of material removal of a much greater orderof magnitude. To machine a billet of combustible pattern material ofexpanded polystyrene whose density is between 1 and 3 pounds per cubicfoot, I adjust the speed, feed and depth of cut of the machine so thatthe rate of material removal for polystyrene will bear substantially thesame ratio to the rate of material removed for steel as the density ofsteel bears to the density of the polystyrene pattern material. Thisrate of material removal may be 400 or 500 times as great as that forsteel, so that a tape-controlled milling machine may cut a pattern in avery short time period. If so rapid rate of material removal should notbe necessary, material may be removed, in any event, at a rate at leastsubstantially 100 times as great as that rate for which the millingmachine is designed to remove steel.

A large diameter cutter, generally designated 32, is used having acentral shaft 33 and plurality of radially projecting bladelike cuttingelements 34 whose vertical cutting edges 35 project from the axis of thespindle 31 a radius f which equals the radius of the large radiusfillets. The program of the machine carves the external verticalsurfaces of the roughly carved article 30 by passing the cutter 32 intothe billet and around the outer sides of the pattern to be formed, withthe cutter axis 31 at a spacing from final contour portions equal to theradius f. In a single pass, it forms all of the vertical outer walls tofinal contour, including the square cut end wall 11 and the sidewalls12. When forming the sidewalls 12, the tape program directs the drivingspindle 31 immediately inward to the axes b at the intersections of thesidewalls 12 and the thin wall sections 13; from such axes b the drivingspindle 31 changes its angular movement direction abruptly, to continueparallel to the thinned wall section 13 to be formed. At each suchabrupt angular change of movement, an internal fillet 14 is formed whoseradius equals that of the milling cutter 32.

Roughly cut hollows 37 within the article 30 are then formed with thesame milling cutter 32, to extend through its entire depth. Machiningthe hollows 37 is commencing in a most unusual manner: the tape programcauses the rotating spindle 31 merely to plunge the cutter 32 axiallydownward to the bottom of the billet, at a suitable distance from innerwalls 38 of the hollows 37. The taped program then directs the spindle31 to carve such walls 38, proceeding to the vertical comer axes wherecomer fillets 39 are to be formed and then abruptly changing itsdirection of angular movement. At the axes where direction of movementis so changed, fillets 39 are formed, with radii equal to the radius fof the cutter 32. Where, as shown, the hollows 37 are rectangular, theprogram should change the angle of spindle movement by 90. The tapeprogram causes the spindle 31 to traverse back and forth until each ofthe hollows 38 is preliminarily carved to the contours shown in FIG. 1.

The physical nature of expanded polystyrene bead material permits adepth of machining out much greater than has been heretofore consideredpossible. I have found it feasible to use a depth of cut substantiallyas large as the entire diameter of the cutter 32, which may be inches ormore. To do this and form a horizontal lower surface, as might be usedin some patterns, the bottom edges 36 of the cutter 32 are formedperpendicular to the spindle 31. Where such great depth of cut is notnecessary, it may in nearly all cases be substantially equal to orgreater than the cutter radius f; and in every case more than 5 timesthe depth of cut feasible for cutting a similar wall from a steel blank.

The final pattern shown in FIG. 2 is characterized by the thinneroutside walls there illustrated, the relatively thin vertical webs 21,and by the stepped thickness leading to the thickened bottom wall 18,with small radius fillets 17, 19 above and below the stop.

To machine the roughly carved billet shown in FIG. 1 into 1 the finalcasting pattern shown in FIG. 2, a second milling cutter generallydesignated 40 is utilized. It includes a central shaft portion 41 withradially extending bladelike cutting elements 42 having vertical cuttingedges 43 whose radial extent g equals the radius of the internal fillets25. In order to form the small radius fillets 17, 19 the lower tip endsof the bladelike cutting elements 42 are arcuately rounded. They providea flat central tip portion 44, which is perpendicular to the shaftportion 41, and arcuately curved outer tip portions 45, whose arcuateradius equals the radii of the smaller radius fillets 17, 19. Thesmaller radius fillets 19 are cut by the outer tip portions 45 while thecentral tip portion 44 cuts the upper surface of the bottom wall 18.

In enlarging the hollows 37 to form the final inner surfaces 20 of thewalls 12 and the vertical web surfaces 21, the procedure used is muchthe same as has heretofore been described for carving the hollows 37.The milling cutter 40 is programmed to plunge its transverse cutter tipportion 44 to the depth at which the bottom wall 18 is to be formed,either directly through previously uncut material of the billet or bydescending first through the hollows 37 to such depth, and then movingsideward therefrom, immediately to a position at which its tip edge 44is at a part of the final contour of the bottom wall 18 so to be carved.The program then directs the cutter 40 to proceed to an axis d of avertical internal fillet 25 and then change its direction of movementabruptly by However, as the cutter 40 moves inward from the verticalwalls 20, 21 the program of the machine moves the spindle 31 axiallydownward, to cut the step in the bottom wall 18 and the small fillets17, also creating large radius comer fillets as seen near the right sideof FIG. 2.

When a flat bottom surface is to be formed inwardly of such small radiusfillets, by successive parallel traverses of the spindle 31, the maximumdepth of cut will be the diameter of the cutter 40 less the radius ofthe outer tip portion 45. Using the expanded polystyrene materialdescribed, bottom surfaces perpendicular to the spindle axis will becleanly cut at this maximum rate of material removal, assuming adequatehorsepower is supplied to the spindle 31.

The present method of machining, at speeds heretofore consideredimpossible, is successful in part because of the unique structure of theself-adhered expanded plastic beads which make up the low-densitypolystyrene material. Only the heads at the final pattern surfaces needbe truly cut; the larger mass of material is removed in effect bygouging.

Thus, the present invention provides for a greatly multiplied rate ofmaterial removal. By removing polystyrene material at a rate whose orderof magnitude bears substantially the same ratio to the rate of materialremoved for steel as the density of steel bears to the density of thepattern material, polystyrene patterns are quickly produced in quantity.If such quantity production does not require the maximum rate ofmaterial removal, it will in any event be at least times as great asthat for steel. The patterns produced by the present invention areprecisely carved, integral, and without objectionable inclusions of waxor glue; in these respects, as well as in lower cost, they are muchpreferable to patterns produced laboriously by pattern makers.

Various modifications, desired for particular uses, will be obvious. Forexample, if instead of the smaller radius fillets 17 19 a straight bevelfillet is desired, the cutter 40 might have beveled tip portions insteadof the arcuate portions 45. The maximum depth of cut would then be thediameter of the cutter less the width of the bevel fillet.

In this application, the term milling machines" includes those machinetools in which cutters are mounted on powered rotating spindles, andwhich provide for relative movement, perpendicular to the spindle axis,between the spindle and the material to be cut. In this category areso-called machining centers" having automatic tool changes controllingtheir tape programs.

The term depth of cut" means the spacing between successive traverses ofthe spindle, measured perpendicular to the spindle axis. Thus where thespindle axis is vertical, as illustrated, the depth of cut is a distancemeasured in a horizontal plane. In referring to milling cutters, theexpression radially projecting bladelike cutting elements is used in thesense that the radial projection of the cutting elements is substantial,and hence capable of making a relatively deep out. As so used, this termincludes cutter elements of convoluted or spiral shape, as well as therelatively flat-bladed cutter elements illustrated. For greatest depthof cut, that is, where the cutter radial projection must be greatest, Iprefer to use such convoluted cutters, so that the pattern material,being carved at a high rate of material removal, will be lifted awayfrom the region of carving.

lclaim:

1. As a new use for a preprogrammed milling machine which follows outautomatically a previously devised program of instructions,

the method of carving casting patterns from pattern material comprisedof self-adhered beads of expanded polystyrene and whose density is lessthan 3 pounds per cubic foot, and which is combustible under a charge ofmolten metal, comprising the steps of providing the machine with arotatable cutter having a cutting edge spaced radially from its axis andhaving the capacity to out such material at more than five times thefeasible depth of cut when machining steel,

chucking in the machine a billet of such material whose size exceedsthat of the casting pattern to be carved therefrom,

feeding to the machine a previously devised program of instructionswhich provides such a linear feed rate and speed of cutter rotation asto achieve a rate of material removal at least 100 times as great asthat for steel, and which directs movement of the cutting edge into thebillet immediately to a position along a portion of the desired finalcontour, and thence directly along said final contour portion, and

operating the machine to carve the billet at such rate of materialremoval according to the program,

whereby to cut to contour those self-adhered beads along the desiredfinal contour and simultaneously to remove the beads making up thematerial outwardly thereof.

2. As a new use for a preprogrammed milling machine,

the method of carving a casting pattern from a pattern material which iscombustible under a charge of molten metal and whose density is lessthan 3 pounds per cubic foot, comprising the steps of providing themachine with a rotatable cutter having a spindle axis of rotation and acutting edge spaced radially outward therefrom, and having the capacityto cut such material at more than five times the feasible depth of cutwhen machining steel,

chucking in the machine a billet of such combustible patternmaterial'whose size exceeds that of the casting pattern to be carvedtherefrom,

providing the machine with a carving program which pro vides such alinear feed rate and speed of cutter rotation as to achieve a rate ofmaterial removal at least 100 times as great as that for steel, andunder which such cutter is directed immediately into the billet, throughsuch excess size, to a point at which its said cutting edge reaches afinal surface portion of the pattern to be carved, and under whichprogram the cutter is then directed along such path that its cuttingedge follows a portion of the final contour of the pattern, and thenoperating the machine to move said cutter according to said program,

whereby the final surface portion of such pattern is provided withoutrepetitive cuts regardless of the amount of excess billet material.

3. The method of carving casting patterns as defined in claim 2,comprising the further steps of providing a machine with a secondrotatable cutter having a cutting edge extending radially outward fromits axis of rotation and following a conformation relative thereto whichdifiers from such first cutter, and

supplementing the carving program so provided for such first cutter witha program for such second cutter, under which it is directed immediatelythrough any excess of billet remaining until its cutting edge reaches afinal surface portion to be carved by said second cutter, and underwhich supplementary program said second cutter is then directed alongsuch path that its cutting edge follows a portion of the final contourof the pattern to be carved by said second cutter, together with thestep of operating the machine to direct said second cutter according tosaid supplementary program, whereby said second cutter carves finalsurface portions according to its conformation directly and withoutrepetitive cuts.

4. As a new use for a tape-controlled milling machine,

the method of carving a casting pattern as described in claim 2, whichhas, as part of its final contour, two surface portions joined by aradially curved internal fillet, com prising the steps defined in claim2, in which the radius of the rotatable cutter so provided equals thatof the fillet, and in which the carving program so provided directs thecutter edge along the final contour of the first of said two surfaceportions to a point at which the cutter axis coincides with the axis ofsaid radially curved internal fillet, and there,

directs the cutter edge, with an abrupt change of direction,

to follow the contour of said second surface portion.

5. As a new use for a preprogrammed. milling machine,

the method of carving a casting pattern from a pattern material which iscombustible under a charge of molten metal and whose density is lessthan 3 pounds per cubic foot, which pattern to be carved includes asurface portion depressed from other portions and extending transverseto the direction at which the axis of a rotatable cutter mayconveniently be applied, comprising the steps of providing the machinewith a rotatable cutter having a spindle axis of rotation, a cuttingedge extending axially and spaced radially outward therefrom, and atip-cutting edge portion extending transverse to its axis of rotation,and having the capacity to out such material at more than five times thefeasible depth of cut when machining steel, then chucking in the machinea billet of such combustible pattern material whose size exceeds that ofthe casting pattern to be carved therefrom,

providing the machine with a carving program which provides such alinear feed rate and speed of cutter rotation as to achieve a rate ofmaterial removal at least times as great as that for steel, and underwhich such cutter is plunged axially into the billet to such depth thatits tip-cutting edge portion reaches the depth of final contour at whichsuch depressed surface portion is to be carved, and under which programthe cutter is then directed through the billet along such path that itscutting edge follows at least part of the final contour of suchtransverse surface portion, and then operating the machine to move saidcutter according to said program,

whereby the tip cutting edge carves such final surface portion at itsdepth so depressed from such other pattern portions, while the toolportion thereabove removes the billet material thereabove.

1. As a new use for a preprogrammed milling machine which follows outautomatically a previously devised program of instructions, the methodof carving casting patterns from pattern material comprised ofself-adhered beads of expanded polystyrene and whose density is lessthan 3 pounds per cubic foot, and which is combustible under a charge ofmolten metal, comprising the steps of providing the machine with arotatable cutter having a cutting edge spaced radially from its axis andhaving the capacity to cut such material at more than five times thefeasible depth of cut when machining steel, chucking in the machine abillet of such material whose size exceeds that of the casting patternto be carved therefrom, feeding to the machine a previously devisedprogram of instructions which provides such a linear feed rate and speedof cutter rotation as to achieve a rate of material removal at least 100times as great as that for steel, and which directs movement of thecutting edge into the billet immediately to a position along a portionof the desired final contour, and thence directly along said finalcontour portion, and operating the machine to carve the billet at suchrate of material removal according to the program, whereby to cut tocontour those self-adhered beads along the desired final contour andsimultaneously to remove the beads making up the material outwardlythereof.
 2. As a new use for a preprogrammed milling machine, the methodof carving a casting pattern from a pattern material which iscombustible under a charge of molten metal and whose density is lessthan 3 pounds per cubic foot, comprising the steps of providing themachine with a rotatable cutter having a spindle axis of rotation and acutting edge spaced radially outward therefrom, and having the capacityto cut such material at more than five times the feasible depth of cutwhen machining steel, chuckIng in the machine a billet of suchcombustible pattern material whose size exceeds that of the castingpattern to be carved therefrom, providing the machine with a carvingprogram which provides such a linear feed rate and speed of cutterrotation as to achieve a rate of material removal at least 100 times asgreat as that for steel, and under which such cutter is directedimmediately into the billet, through such excess size, to a point atwhich its said cutting edge reaches a final surface portion of thepattern to be carved, and under which program the cutter is thendirected along such path that its cutting edge follows a portion of thefinal contour of the pattern, and then operating the machine to movesaid cutter according to said program, whereby the final surface portionof such pattern is provided without repetitive cuts regardless of theamount of excess billet material.
 3. The method of carving castingpatterns as defined in claim 2, comprising the further steps ofproviding a machine with a second rotatable cutter having a cutting edgeextending radially outward from its axis of rotation and following aconformation relative thereto which differs from such first cutter, andsupplementing the carving program so provided for such first cutter witha program for such second cutter, under which it is directed immediatelythrough any excess of billet remaining until its cutting edge reaches afinal surface portion to be carved by said second cutter, and underwhich supplementary program said second cutter is then directed alongsuch path that its cutting edge follows a portion of the final contourof the pattern to be carved by said second cutter, together with thestep of operating the machine to direct said second cutter according tosaid supplementary program, whereby said second cutter carves finalsurface portions according to its conformation directly and withoutrepetitive cuts.
 4. As a new use for a tape-controlled milling machine,the method of carving a casting pattern as described in claim 2, whichhas, as part of its final contour, two surface portions joined by aradially curved internal fillet, comprising the steps defined in claim2, in which the radius of the rotatable cutter so provided equals thatof the fillet, and in which the carving program so provided directs thecutter edge along the final contour of the first of said two surfaceportions to a point at which the cutter axis coincides with the axis ofsaid radially curved internal fillet, and there, directs the cutteredge, with an abrupt change of direction, to follow the contour of saidsecond surface portion.
 5. As a new use for a preprogrammed millingmachine, the method of carving a casting pattern from a pattern materialwhich is combustible under a charge of molten metal and whose density isless than 3 pounds per cubic foot, which pattern to be carved includes asurface portion depressed from other portions and extending transverseto the direction at which the axis of a rotatable cutter mayconveniently be applied, comprising the steps of providing the machinewith a rotatable cutter having a spindle axis of rotation, a cuttingedge extending axially and spaced radially outward therefrom, and atip-cutting edge portion extending transverse to its axis of rotation,and having the capacity to cut such material at more than five times thefeasible depth of cut when machining steel, then chucking in the machinea billet of such combustible pattern material whose size exceeds that ofthe casting pattern to be carved therefrom, providing the machine with acarving program which provides such a linear feed rate and speed ofcutter rotation as to achieve a rate of material removal at least 100times as great as that for steel, and under which such cutter is plungedaxially into the billet to such depth that its tip-cutting edge portionreaches the depth of final contour at which Such depressed surfaceportion is to be carved, and under which program the cutter is thendirected through the billet along such path that its cutting edgefollows at least part of the final contour of such transverse surfaceportion, and then operating the machine to move said cutter according tosaid program, whereby the tip cutting edge carves such final surfaceportion at its depth so depressed from such other pattern portions,while the tool portion thereabove removes the billet materialthereabove.